This invention relates to video image keying systems and methods.
Keying systems are widely used for picture editing and composition in television and are used to key a foreground image into a background image, For example a keying system may be used for keying letters or other characters forming a caption into a picture to provide information relating to the subject matter of the picture. In such a case video signals representing the characters, as selected by the operator, are generated by a source such as a character generator, for example the character generator sold by the assignee hereof Quantel, Ltd. of Newbury, England under the trade mark "CYPHER". When a character is selected, the generator not only provides the video signals representing the character which may be regarded as foreground signals F, but it also provides internal control signals K which are used to key the character or foreground video signals over other video signals representing a background matte B. Normally the background matte B is a uniform black. The resulting combined signal is output from the source for further processing by other television image apparatus. The control signals K are also used within the source to key video signals representing white (value 1) over video signals representing black (value 0) to produce a key signal .alpha. which is output from the source for external use together with the combined signal. Thus, the output of the character generator comprises the video signal representation V of the foreground F keyed over the background B together with the external key . The external key .alpha. is used subsequently in external circuitry as will be described in greater detail hereinafter.
At present it is usual to perform the keying of the foreground F over the background matte B within the source character generator using a linear interpolating circuit in which the internal control signals K function as an interpolating coefficient. The video V which is the result of the interpolation can then be represented by EQU V=F.K+B(1-K) (1)
It is also usual for the control signals K to have a maximum value, which may be 1 or less than 1, for pixels lying within the character represented by the video signals F and to have value 0 for pixels within the background matte B, except for a zone round the boundary of the character over which the control signals K decline gradually from the maximum value, e.g. 1, to 0 to produce a soft edge between the character F and the background matte B. As is well known, this so-called soft edge keying is employed to reduce aliasing which would otherwise arise between character and background. The maximum value of K will be less than 1 where a foreground image F is to be combined transparently with the background matte B. Thus the coefficient K in the above expression for V may have values in the range: EQU 0.ltoreq.K.ltoreq.1
and it will be understood that similar soft edging will also appear in the external key signal .alpha.. In the accompanying drawings FIG. 1(A) and 1(B) together illustrate a character generator or source of the above described form, conditioned to generate an output video as represented by the above expression for V and also to generate the above mentioned external key .alpha.. All the Figures in the drawings are in the form of schematic functional diagrams in which different circuits elements are represented by blocks identified by functional symbols denoting the nature of the respective circuit elements. Specific explanation of individual functional units in the drawings is considered to be unnecessary for an understanding of the invention by those possessed of the appropriate skills.
Returning to FIG. 1(A) it will be appreciated that where K=0 pixels in the video image V will relate solely to the matte background B, where K=1 pixels in the video image V will relate solely to the foreground character F and that for pixels within the soft edges, where K is between 1 and 0, the output video V will include a contribution both from F and from the background B. In the special case where the background matte is black of value B=0, then the term B(1-K) will remain 0 regardless of the value of K, but the term F.K will diminish with K at the soft edges, thereby "darkening" the character, in effect increasing the contribution from black. It will also be appreciated from FIG. 1(B) that the value of .alpha. will track that of K, being non-zero where K is non-zero.
The external key .alpha. is used by external circuitry to key the video signal V output from the source into video signals representing a picture which may be regarded as arbitrary external background Bg. In this way a resultant picture R is produced comprising the background picture Bg and the foreground character F.
The external keying of the video V into the arbitrary external background Bg has hitherto usually been performed by another linear interpolation process as illustrated in FIG. 1(c) to produce the result: EQU R=.alpha.V+Bg (1-.alpha.)
Bearing in mind that from equation (1) hereinabove V =F.K+B(1-K) then the above equation for R shows that any contribution of the original background matte B in the signal V is multiplied by .alpha.. As mentioned hereinabove pixels within the soft edges are darkened towards black by the keying performed within the character source. In other words pixels in the soft edge output from the character source include an inseparable contribution from the black background of the matte. Therefore, the result signal R from the external interpolator includes the black background contribution in the soft edges thereof in addition to the external background Bg and this black contribution is therefore a spurious signal which may be seen as halos or shadows in the result image R. While these unwanted artifacts are usually feint they are nevertheless visible and can be disturbing. Similar problems can occur in other systems where two or more keying operations are required over the same area of a picture, for example in systems such as described in commonly assigned British Patent No. 2,113,950 (corresponding U.S. Pat. No. 4,602,286), or in systems in which different foreground objects can be internally keyed over two copies of the same background picture and an output produced by internally cross fading between one stamped copy and the other.